This invention relates to a carburetor for a two stroke engine and more particularly to a carburetor which provides a scavenging air supply independently of a fuel and air mixture.
In a conventional crankcase compression type two stroke internal combustion engine, a fuel and air mixture is supplied to the engine crankcase through a fuel and air mixing passage in the carburetor. The fuel and air mixture in the crankcase is pressurized in the crankcase during a portion of the stroke of a piston of the engine and is supplied through a scavenging or transfer port to a combustion chamber of the engine cylinder to assist the scavenging of combustion gases which remain in the cylinder and to provide a fresh fuel and air mixture for the next combustion event. Undesirably, a portion of the fresh fuel and air mixture provided to the combustion chamber escapes through the exhaust port with the exhaust gas thereby increasing the hydrocarbon content of the engine emissions and reducing the fuel efficiency of the engine.
At least in part to reduce engine hydrocarbon emissions and improve fuel efficiency, it has been proposed to provide a scavenging air supply to the engine through a second bore in the carburetor. A throttle valve may be used to control fluid flow through the fuel and air mixture passage and an air valve to control the scavenging air flow through the second bore. An example of a conventional carburetor having the throttle valve and air valve provided in a carburetor body is shown in Japanese Patent No. 9,268,918
However, at least at engine idle operation, the scavenging air supply is undesirable as it provides too much air to the engine causing an undesirably lean fuel and air mixture in the engine and poor engine performance, stability and acceleration. Therefore, selective control of the scavenging air supply relative to the fuel and air mixture supply, is needed. However, in a carburetor wherein the throttle valve and the air valve are barrel type valves formed integrally with each other, in order to control the opening time of the air valve relative to the throttle valve, the throttle valve and the air valve must have different diameters or there must be a difference in the extent to which the valves open. Forming the valves of different diameter increases the cost to make the valves and the cost to manufacture the carburetor to accommodate the differently sized valves. Limiting the extent to which the air valve opens is also undesirable because at wide open throttle operating conditions, it is desirable to provide as much air as possible to the engine to assist in scavenging exhaust gases, to increase the mixing of fuel and air in the combustion chamber and to support combustion of the fuel and air mixture in the combustion chamber.
A carburetor for providing a fuel and air mixture and a scavenging air supply to a two stroke internal combustion engine has a throttle valve received in a fuel and air mixing passage and connected to a throttle drive for controlling engine operation and an air valve received in an air passage and selectively operably connected to the throttle valve by a lost motion coupling. Preferably, the throttle valve and the air valve are barrel-type valves each comprising an opening formed through a shaft extending transversely through and rotatable within an associated bore of the carburetor body. The lost motion coupling may comprise a pair of selectively meshed gears consisting of a first partial gear connected to a shaft of the throttle valve for co-rotation with the throttle valve shaft, and a second partial gear connected to a shaft of the air valve for co-rotation with the air valve shaft. A gear ratio of the gear pair is set in such a manner that the first partial gear does not mesh with the second partial gear until the throttle valve has rotated to a predetermined angle or partially open position from an idle position and the throttle valve and the air valve are simultaneously operable at a fully opened position to permit substantially unrestricted fluid flow through their respective passages in the carburetor body.
To increase the engine operating speed or power from engine idle operation, a throttle valve lever is rotated against a spring biasing it to its idle position by means of an operating lever through a remote cable, to ether open the throttle valve from its idle position towards its wide open position. According to the invention, when the first partial gear has rotated a predetermined angle, the first partial gear abuts against or initially meshes with the second partial gear which is connected to a shaft of the air valve and continued opening of the throttle valve causes the first partial gear to drive the second partial gear for rotation. This rotation of the second partial gear rotates the air valve from its fully closed position toward its fully open position. The first partial gear continues to rotate the second partial gear to increase the opening degree of the air valve until it reaches its fully opened position preferably at the same time the throttle valve reaches its wide open position. In this manner the throttle valve and the air valve can be simultaneously opened, by providing the throttle valve and the air valve as separate members connected to the gears which have different outer diameters, and by selecting shapes and numbers of teeth of the gears, and determining an appropriate gear ratio.
Objects, features and advantages of the invention include providing a carburetor for supplying a fuel and air mixture to an engine and selectively supplying a scavenging air supply to the engine which terminates the scavenging air supply at low speed and low load engine operation, permits timing of the initial opening of the air valve relative to the throttle valve of the carburetor to be freely set, selectively interconnects the throttle valve and air valve, permits at least some relative movement between the throttle valve and air valve, may have a throttle valve and an air valve which need not be different from each other in diameter or in the degree or extent to which they open in order to retard the opening time of the air valve and inhibit or prevent providing the scavenging air supply at least at engine idle operation, does not interconnect the throttle valve and air valve with a lever and link arrangement, avoids providing too much air to the engine, improves engine operation, reduces engine emissions, increases engine fuel efficiency, is of relatively simple design and economical manufacture and assembly, reliable and in service has a long useful life.